Pakistan's president has acknowledged for the first time that his security forces were left out of a US operation to kill Osama bin Laden.

Writing in the Washington Post, president Asif Ali Zardari defended his country against accusations it did not do enough to track down the Al Qaeda leader.

"Although the events of Sunday were not a joint operation, a decade of cooperation and partnership between the United States and Pakistan led up to the elimination of Osama bin Laden as a continuing threat to the civilised world," he wrote in an opinion piece for the US newspaper.

Underneath a headline reading "Pakistan did its part," he added:"we in Pakistan take some satisfaction that our early assistance in identifying an Al Qaeda courier ultimately led to this day."

The CIA is now sifting through what has been described as a boatload of intelligence, retrieved from bin Laden's compound in Abbottabad, a hillside retreat popular with retired Pakistani generals just a few hours drive from Islamabad.

The discovery of the world's most hunted terrorist so close to the Pakistani capital seems to vindicate suspicions that Al Qaeda found sanctuary in Pakistan, which has received billions of dollars in exchange for pursuing terrorists.

US counter-terrorism chief John Brennan says it is "inconceivable" bin Laden could have lived in such a conspicuous, sprawling compound so close to Islamabad without a support network inside the country.

"I am not going to speculate about what type of support he might have had on an official basis inside of Pakistan," he said.

"We are closely talking to the Pakistanis right now and, again, we are leaving open opportunities to continue to pursue whatever leads might be out there."

Mr Brennan said US officials did not notify Pakistan of the raid until its helicopters exited Pakistani airspace with bin Laden's remains.

'Baseless speculation'

Mr Zardari provided no detailed explanation on how bin Laden managed to live in Abbottabad for years undetected.

But he criticised US press coverage suggesting that Pakistan "lacked vitality" in pursuing terrorists.

"He was not anywhere we had anticipated he would be, but now he is gone," he wrote.

"Some in the US press have suggested that Pakistan lacked vitality in its pursuit of terrorism or, worse yet, that we were disingenuous and actually protected the terrorists we claimed to be pursuing.

"Such baseless speculation may make exciting cable news, but it doesn't reflect fact. Pakistan had as much reason to despise Al Qaeda as any nation.

"The war on terrorism is as much Pakistan's war as it is America's. And though it may have started with bin Laden, the forces of modenity remain under serious threat."

Former Pakistani leader Pervez Musharraf also rejected the notion of any collusion between bin Laden and the authorities.

"I would say it is certainly ... an intelligence failure," he said.

"Since you are blaming Pakistan, let me also blame the United States, to an extent. It is also a failure of the CIA, may I say, who were very much in total co-operation with Pakistan.

"But any aspersions that the Pakistan military or the intelligence knew about him and that he was staying there this is absolutely wrong."

But despite Mr Musharraf's protestations, the chairman of the US Senate Homeland Security Committee, Senator Joe Lieberman, has promised tough questions about the US ally when officials come before the Senate this week, and the ranking Republican Member of the Committee, Susan Collins agrees.

"It's very difficult for me to understand how this huge compound could be built in a city just an hour north of the capital of Pakistan, in a city that contained military insulations, including the Pakistani Military Academy and that it did not arouse tremendous suspicious," she said.

Former defence minister Joel Fitzgibbon says the Federal Government needs to know if Pakistan was complicit in harbouring bin Laden, in order to understand the prospects for success in Afghanistan.

"We need to be reassured if our troops are going to continue to put their lives on the line in Afghanistan that one of our key partners, Pakistan, is going to be absolutely committed to the project," he said.

Meanwhile, Pakistan is bracing for a backlash, fearful of a wave of revenge attacks.

Pakistan's main Taliban faction reacted angrily to the announced death of the Al Qaeda number one, promising to "avenge his death and launch attacks against American and Pakistani governments and their security forces".

Pakistan has beefed up security across major cities, diplomatic installations and around the site of the killing in Abbottabad.

More troops were deployed in Islamabad to safeguard government offices and the city's diplomatic enclave, while in Lahore and Karachi, the two biggest cities, extra road blocks and barbed wire were laid around sensitive buildings.

The area in Pakistan where bin Laden was found is usually a placid place, but local residents like Naeem Mahmood worried that it will not stay that way.

"Yeah I'm really worried about the community because it's going to take a lot from the intelligence agencies, army to take the fear out because all the community and especially us and everyone around here is in a total state of shock and they're in fear," he said.

"This was the most safest place in Pakistan and I don't think it's going to be like this.

"There will be repercussions and Al Qaeda is going to plan some revenge attacks particularly in this place and all over Pakistan."